Such a through-plating of a printed circuit board serves, for example, to establish an electrically conductive connection between a conductive track running on the first surface and a conductive track running on the second surface, thereby enabling electronic components to be fixed to both surfaces, which electronic components can be inter-connected with one another in accordance with a circuit to be realized.
Through-platings are normally holes which are made in the printed circuit board at points which are pre-determined by the layout of the circuit to be realized, the inner sides of said holes being metallized.
As long as only components having connecting wires or legs (resistors, capacitors, coils, transformers, diodes, transistors, integrated circuits, etc.) were intended to be fixed to the printed circuit boards, it was possible to insert the connecting wires or legs directly into the metalized holes and solder them therein. Today's components, however, are almost exclusively SMD components. (The acronym SMD is formed from surface mounted device).
In contrast to components having connecting wires or legs, SMD components only have connecting pads. The latter are soldered in a planar manner on to conductive tracks of printed circuit boards. The holes which are not closed off by the metallization, in other words are open, are not closed off by this soldering and remain open.
If multilayer printed circuit boards with through-platings which pass only through some of the layers and are covered over by outer layers are to be produced, the air remaining in the open holes as a result of the latter being covered over would expand in the event of a temperature increase and might give rise to internal cracks.